1. Field of the Invention
The present invention relates generally to implantable orthopedic prostheses, and more particularly to an instrument for disassembling a head and acetabular component of a bipolar orthopedic hip prosthesis.
2. Background Information
One known configuration of an implantable hip prosthesis involves a femoral component having a stem for implantation in the proximal femur and a conically tapered neck on which is received a spherical head having a corresponding conically tapered bore. The male taper of the neck and the female taper of the head are selected to provide a taper locking interference fit when mutually engaged. An acetabular component is provided for use with the femoral component. The acetabular component can be of the type having an outer shell with a smooth, spherical outer surface for receipt within the natural acetabulum, and a bearing insert having a smooth, spherical inner surface for receiving the head of the femoral component. The outer shell is usually constructed of a biocompatible metal having a highly polished surface that moves freely within the natural acetabulum. The bearing insert is usually constructed of a biocompatible, low friction plastic material such as high density polyethylene. The smooth inner surface of the bearing insert moves freely relative to the femoral head. This type of acetabular component is sometimes referred to as a "bipolar" prosthesis due to the freedom of movement which exists between the femoral head and bearing insert, and between the smooth outer shell and the natural acetabulum.
To provide the implanting surgeon with maximum flexibility in fitting a bipolar hip prosthesis to a particular patient, the various components of the hip prosthesis are often provided in a range of sizes for selection and assembly at the time of implantation. For example, the femoral heads might be provided in a single diameter, but with the female taper disposed at various depths within the head to vary the offset of the head from the femoral stem. Furthermore, the acetabular components might be provided with an inner bearing diameter sized to match the common femoral head diameter, but with various diameters of the outer shell being available to match the patient's acetabulum. Because of the number of different possible combinations of femoral heads and acetabular components, it is preferred to assemble the selected head to the selected acetabular component at the time of implantation. The design of the bearing insert therefore requires that the femoral head can be inserted therein with relative ease, and yet be constrained therein against dislocation and separation under the forces to which the hip joint is commonly exposed.
One configuration for an acetabular component that permits ease of assembly and yet constrains the femoral head against separation involves a poly bearing insert that provides somewhat more than hemispherical coverage of the femoral head and that has an opening to the bearing cavity that is somewhat smaller in diameter than the diameter of the femoral head. Such a bearing insert undergoes temporary elastic deformation as the femoral head is forced into the bearing cavity, followed by a return to its initial configuration which provides constraint of the femoral head therein.
Occasionally, after the implanting surgeon has made an initial selection of a femoral head and an acetabular component and has assembled them together, it will subsequently be determined that a different component size would be preferable. In such situations it is desirable that appropriate instrumentation be available to disassemble the femoral head and acetabular component without risk of damage to the bearing insert or the highly polished surfaces of the femoral head and the outer shell.